A growing global trend is deregulation of telecommunications systems, particularly telephone call switching systems. With deregulation, new common carriers are entering the telephone call switching business. These carriers, as well as established carriers, are seeking to extend their services to new customers, markets, and nations.
However, a difficult problem facing such carriers is interconnecting their equipment with systems available in these new markets and nations. The new customers, markets and nations often use signaling systems and protocols that are different from the protocols that are customary or popular in North America.
In telephony, "signaling" refers to the exchange of call control information between the phone switch and the caller. The purpose of a signaling system is to transfer control information, or signaling units, between elements in a telecommunications system.
Early signaling systems carried the control signals on the same circuit as the user traffic. For example, older in-band signaling systems use this approach. Newer signaling systems, such as Common Channel Signaling System 7 (SS7), use a separate channel for signaling information. These systems are called common channel signaling (CCS) systems because a separate (common) channel is used for signaling. Some call this approach "out of band" signaling and it is preferred to in-band signaling because it is more efficient and robust. For example, out of band signaling offers faster call setup times, more efficient use of voice circuits, support for intelligent network services, and improved control over fraudulent network usage.
Two types of out-of-band signaling exist today. SS7 is a global standard for telecommunications defined by the International Telecommunications Union (ITU). The standard defines the procedures and protocol by which network elements in the public switched telephone network exchange information over a digital signaling network to effect wireless (cellular) and wireline call setup, routing and control. The ITU definition of SS7 allows for national variants such as the ANSI and Bellcore standards used in North America.
SS7 is an example of the first type, physical out-of-band signaling, in which a separate physical channel is used for signaling. SS7 is usually, but not necessarily, deployed as a separate network within the complete telephone network architecture for the purpose of establishing and terminating telephone calls. SS7 messages are exchanged between network elements over bidirectional channels called signaling links.
ISDN is an example of physical in-band, logical out-of-band signaling. In this approach, signaling and user traffic share the same physical transmission medium, but part of the channel capacity is reserved only for signaling traffic. The remainder of the bandwidth is reserved for user traffic, such as the telephone call. ISDN uses the term B channel to describe the user channel, and the term D channel to describe the signaling channel.
Other signaling schemes can be used with T1 and E1 circuits. For example, in some forms of CAS every T1 frame or E1 frame is a voice/bearer/traffic channel in which signaling is carried in "stolen" bits and DTMF tones are transmitted in the voice/bearer/traffic channel.
Further information on such signaling systems is provided in U. Black, "ISDN & SS7: Architectures for Digital Signaling Networks" (Upper Saddle River, N.J.: Prentice Hall PTR, 1997).
Generally, Common Channel Signaling (CCS) is employed to pass signaling information in an SS7 network. However, other methods can be used to pass more information in the T1/E1 frame, such as Channel Associated Signaling (CAS). Channel Associated Signaling uses one bit (called the F bit) per frame to pass special data. Channel Associated Signaling information is included within each voice channel on a digital link. For example, DTMF tones may be passed in-band across the voice channel.
Common Channel Signaling systems and signals are inherently incompatible with Channel Associated Signaling systems and signals. An attempt to connect a call originating from a Common Channel Signaling system to a Channel Associated Signaling system can lead to unpredictable results at either the calling end of the circuit or the called end of the circuit.
Based on the foregoing, there is a clear need in the field to resolve interworking issues between networks using Channel Associated Signaling (CAS) and Common Channel Signaling (CCS) systems.
In particular, there is a need for a system that can carry out conversion of signals in the CAS protocol to signals using a CCS protocol.
There is also a need for a system that can manage a large number of Channel Associated Signaling ports, and that can be expanded to accommodate an ever-increasing number of ports.